Method of manufacture of chalcogenide-based photovoltaic cells

ABSTRACT

The invention is a method of forming a cadmium sulfide based buffer on a copper chalcogenide based absorber in making a photovoltaic cell. The buffer is sputtered at relatively high pressures. The resulting cell has good efficiency and according to one embodiment is characterized by a narrow interface between the absorber and buffer layers. The buffer is further characterized according to a second embodiment by a relatively high oxygen content.

FIELD OF THE INVENTION

This invention relates to a method of manufacture of chalcogenide-basedphotovoltaic cells and particularly to a method of forming a bufferlayer in such cells and the cells made by this method.

BACKGROUND OF THE INVENTION

Photovoltaic cells can be made using p-type chalcogenide based materialsas absorber layers which convert incident light or radiation toelectrical energy. These p-type chalcogenides are typically selenides,sulfides or sulfide selenides of at least one, and more typically atleast two or three of the following metals: Cu, In, Ga, Al (referred toherein as CIS, CISS, CIAS, CIASS, CIGS, CIGSS, or CIAGSS depending uponthe combination of elements used). Using a CdS based buffer layer nearor adjacent to the p-type chalcogenide is also known.

It is known that CdS layers can be formed on various substrates bychemical bath deposition, physical vapor deposition or sputtering. Seee.g. Abou-Ras et al (Thin Solid Films 480-481 (2005) 118-123) and5,500,055. Abou-Ras specifically looked at the effect of depositionmethod comparing CBD deposition to PVD deposition and observed that CBDdeposition of the CdS created more efficient cells compared with cellsmade with PVD. Abou-Ras proposes that the lack or decrease ofinterdiffusion at the CIGS-CdS interface in the case of PVD depositedcells is a reason for their decreased efficiencies.

SUMMARY OF THE INVENTION

Applicants have surprisingly found that sputtering CdS onto the p-typechalcogenide at relatively high pressures in a substantially inertatmosphere leads to less interdiffusion with the underlying chalcogenideabsorber than does sputtering at the traditional low pressure sputteringconditions but also leads to higher cell efficiencies than does thetraditional low pressure sputtering. This is particularly unexpected inview of Abou-Ras's teaching.

Thus, according to one embodiment, the invention is a method comprising

-   -   forming a chalcogenide-based absorber layer on a substrate,    -   forming a buffer layer comprising cadmium and sulfur on the        absorber by sputtering in an inert atmosphere at a working        pressure of from 0.08 to 0.12 mbar (0.06 to 0.09 torr or 8-12        Pa).

This invention is also a photovoltaic cell made by the preceding method.The cells made by this method are characterized by interdiffusionbetween the CdS and absorber layer. The interdiffusion interface regionis defined at the absorber region by the point at which the atomicfraction of cadmium exceeds 0.05 in energy dispersive x-ray spectroscopy(EDS) scans of cross section of the cell and defined at the bufferregion by the point at which the atomic fraction of indium and seleniumis less than 0.05 and preferably the atomic fraction of copper is lessthan 0.10. The atomic fraction is based on total atomic amounts ofcopper, indium, gallium, selenium, cadmium, sulfur, and oxygen. Thegrain size of the cadmium sulfide grains preferably is less than 50 nm,more preferably less than 30 nm, and most preferably less than 20 nm.Thus, according to one embodiment the invention is a photovoltaic cellcomprising a backside electrode (also referred to herein as a backsideelectrical contact or backside electrical collector), achalcogenide-based absorber in contact with the backside electrode, acadmium sulfide based buffer layer on the absorber, a transparentconductive layer located at the opposite side of the buffer layer fromthe absorber layer, an electrical collector on the transparentconductive layer, wherein the cell has an interface between the absorberand the buffer of less than 10 nm thickness and the buffer preferablyhas an average grain size of less than 50 nm.

Surprisingly, although the cadmium sulfide preferably is sputtered in aninert environment, the buffer layer of the cells made by this inventionhas a significant amount of oxygen. Thus, according to anotherembodiment this invention is a photovoltaic cell comprising a backsideelectrode, a chalcogenide based absorber in contact with the backsideelectrode, a cadmium sulfide based buffer layer on the absorber, atransparent conductive layer located at the opposite side of the bufferlayer from the absorber layer, an electrical collector on thetransparent conductive layer, wherein the atomic fraction of oxygen inthe cadmium sulfide based buffer layer is at least 0.20.

Also, surprisingly the thickness of the buffer layer may be very lowwhile still yielding effective cells while providing the additionalbenefit of low cadmium leachate from the cell. Thus, according to yetanother embodiment the invention is a photovoltaic cell comprising abackside electrode, a chalcogenide-based absorber in contact with thebackside electrode, a cadmium sulfide based buffer layer on theabsorber, a transparent conductive layer located at the opposite side ofthe buffer layer from the absorber layer, an electrical collector on thetransparent conductive layer, wherein the thickness of the buffer layerno greater than 30 nm, preferably no greater than 20 nm, and mostpreferably no greater than 15 nm. The amount of Cadmium that leachesfrom the article under the USEPA Toxicity Characteristic LeachingProcedure Test 1311 (1992) is no greater than 1 mg/l (i.e. 1 mg ofcadmium per liter of leachant solution as specified in the protocol)preferably no greater than 0.8 mg/l, and most preferably no greater than0.7 mg/l.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a cross section of a representative cellaccording to the present invention.

FIG. 2 shows plots of the atomic fraction of a cell made by the presentmethod by sputtering at 0.1 millibar.

FIG. 3 shows the atomic fraction of a comparative cell made bysputtering at 0.002 millibar.

DETAILED DESCRIPTION OF THE INVENTION

The method of this invention includes forming a chalcogenide basedabsorber layer (absorbs electromagnetic radiation at relevantwavelengths and converts to electrical energy) on a substrate and thenforming a cadmium sulfide buffer layer on that absorber. When the methodis used to form a photovoltaic device, additional layers as are known inthe photovoltaic arts also will typically be added. For example, thesubstrate will typically include or bear backside electrical contacts. Atransparent conductive layer will be found above the buffer and aelectrical collector system (e.g. a grid) will typically be locatedabove the transparent conductive layer. An optional window layer may beused and protective layers may be applied over the transparentconductive layer and/or the electrical collector. With the exception ofthe cadmium sulfide based buffer layer, these additional layers may beformed by any method known in the art.

FIG. 1 shows one embodiment of a photovoltaic article 10 that may bemade by processes of the invention. This article 10 comprises asubstrate incorporating a support 22, a backside electrical contact 24,and a chalcogenide absorber 20. The article 10 further includes a bufferregion 28 incorporating an n-type chalcogenide composition of thepresent invention, an optional front side electrical contact windowregion 26, a transparent conductive region 30, a collection grid 40, andan optional barrier region 34 to help protect and isolate the article 10from the ambient. Each of these components is shown in FIG. 1 asincluding a single layer, but any of these independently can be formedfrom multiple sublayers as desired. Additional layers (not shown)conventionally used in photovoltaic cells as presently known orhereafter developed may also be provided. As used occasionally herein,the top 12 of the cell is deemed to be that side which receives theincident light 16. The method of forming the cadmium sulfide based layeron the absorber can also be used in tandem cell structures where twocells are built on top of each other each with an absorber that absorbsradiation at different wavelengths.

The support 22 may be a rigid or flexible substrate. Support 22 may beformed from a wide range of materials. These include glass, quartz,other ceramic materials, polymers, metals, metal alloys, intermetalliccompositions, paper, woven or non-woven fabrics, combinations of these,and the like. Stainless steel is preferred. Flexible substrates arepreferred to enable maximum utilization of the flexibility of the thinfilm absorber and other layers.

The backside electrical contact 24 provides a convenient way toelectrically couple article 10 to external circuitry. Contact 24 may beformed from a wide range of electrically conductive materials, includingone or more of Cu, Mo, Ag, Al, Cr, Ni, Ti, Ta, Nb, W combinations ofthese, and the like. Conductive compositions incorporating Mo arepreferred. The backside electrical contact 24 may also help to isolatethe absorber 20 from the support 22 to minimize migration of supportconstituents into the absorber 20. For instance, backside electricalcontact 24 can help to block the migration of Fe and Ni constituents ofa stainless steel support 22 into the absorber 20. The backsideelectrical contact 24 also can protect the support 22 such as byprotecting against Se if Se is used in the formation of absorber 20.

The chalcogenide absorber 20 preferably incorporates at least one p-typeGroup 16 chalcogenide, such as Group 16 selenides, sulfides, andselenides-sulfides that include at least one of copper, indium, and/orgallium. In many embodiments, these materials are present inpolycrystalline form. Advantageously, these materials exhibit excellentcross-sections for light absorption that allow absorber 20 to be verythin and flexible. In illustrative embodiments, a typical absorberregion 20 may have a thickness in the range from about 300 nm to about3000 nm, preferably about 1000 nm to about 2000 nm.

Representative examples of such p-type chalcogenides absorbers areselenides, sulfides, tellurides, and/or combinations of these thatinclude at least one of copper, indium, aluminum, and/or gallium. Moretypically at least two or even at least three of Cu, In, Ga, and Al arepresent. Sulfides and/or selenides are preferred. Some embodimentsinclude sulfides or selenides of copper and indium. Additionalembodiments include selenides or sulfides of copper, indium, andgallium. Aluminum may be used as an additional or alternative metal,typically replacing some or all of the gallium. Specific examplesinclude but are not limited to copper indium selenides, copper indiumgallium selenides, copper gallium selenides, copper indium sulfides,copper indium gallium sulfides, copper gallium sulfides, copper indiumsulfide selenides, copper gallium sulfide selenides, copper indiumaluminum sulfide, copper indium aluminum selenide, copper indiumaluminum sulfide selenide, copper indium aluminum gallium sulfide,copper indium aluminum gallium selenide, copper indium aluminum galliumsulfide selenide, and copper indium gallium sulfide selenides. Theabosber materials also may be doped with other materials, such as Na,Li, or the like, to enhance performance In addition, many chalcogenmaterials could incorporate at least some oxygen as an impurity in smallamounts without significant deleterious effects upon electronicproperties.

One preferred class of CIGS materials may be represented by the formula

Cu_(a)In_(b)Ga_(c)Al_(d)Se_(w)S_(x)Te_(y)Na_(z)   (A)

Wherein, if “a” is defined as 1, then:

-   -   “(b+c+d)/a”=1.0 to 2.5, preferably 1.0 to 1.65    -   “b” is 0 to 2, preferably 0.8 to 1.3    -   “c” is 0 to 0.5, preferably 0.05 to 0.35    -   “d” is 0 to 0.5, preferably 0.05 to 0.35, preferably d=0    -   “(w+x+y)” is 2 to 3, preferably 2 to 2.8    -   “w” is 0 or more, preferably at least 1 and more preferably at        least 2 to 3    -   “x” is 0 to 3, preferably 0 to 0.5    -   “y” is 0 to 3, preferably 0 to 0.5    -   “z” is 0 to 0.5, preferably 0.005 to 0.02

The absorber 20 may be formed by any suitable method using a variety ofone or more techniques such as evaporation, sputtering,electrodeposition, spraying, and sintering. One preferred method isco-evaporation of the constituent elements from one or more suitabletargets, where the individual constituent elements are thermallyevaporated on a hot surface coincidentally at the same time,sequentially, or a combination of these to form absorber 20. Afterdeposition, the deposited materials may be subjected to one or morefurther treatments to finalize the absorber properties.

Optional layers (not shown) may be used on substrate in accordance withconventional practices now known or hereafter developed to help enhanceadhesion between backside electrical contact 24 and the support 22and/or between backside electrical contact 24 and the absorber region20. Additionally, one or more barrier layers (not shown) also may beprovided over the backside of support 22 to help isolate device 10 fromthe ambient and/or to electrically isolate device 10.

The buffer region 28 is cadmium sulfide based material deposited on theabsorber 20 by sputtering at pressures of at least 0.08 millibar (0.06torr, 8 Pa), more preferably at least 0.09 millibar (0.067 torr, 9 Pa),and most preferably about 0.1 millibar (0.075 torr, 10 Pa) and nogreater than 0.12 millibar (0.09 torr, 12 Pa), more preferably nogreater than 0.11 millibar (0.083 torr, 11 Pa). Preferably theatmosphere is inert or a sulfur containing gas, but is most preferablyinert.

During such deposition approach, the substrate is typically fixed to orotherwise supported upon a holder within the chamber such as by grippingcomponents, or the like. However, the substrate may be oriented andaffixed by a wide variety of means as desired. The substrate may beprovided in the chamber in a manner such that the substrate isstationary and/or non-stationary during the treatment. In someembodiments, for instance, the substrate can be supported on a rotatablechuck so that the substrate rotates during the deposition.

One or more targets are operably provided in the deposition system. Thetargets are compositionally suitable to form the desired cadmium sulfidecomposition. For instance, to form n-type cadmium sulfide, a suitabletarget has a composition that includes cadmium and sulfur-containingcompounds, and is preferably 99% pure cadmium and sulfur. Alternatively,a cadmium target can be used in the presence of a sulfur containing gas.The resulting film is preferably at least 10 nanometers (nm), morepreferably at least 15 nm and is preferably up to about 200 nm, morepreferably up to 100 nm, yet more preferably up to 30 nm, still morepreferably up to 20 nm and most preferably not more than 15 nm Since thebuffer functions effectively at these yet more, still more and mostpreferred very thin layers, the amount of cadmium in the cell isrelatively low compared to prior art cells. These cells have the addedbenefit of a low cadmium leachate amount.

While a small amount of interdiffusion in cells made by the method ofthis invention may occur it is significantly less than is found in whenthe atmosphere for sputtering is lower pressure. The interdiffusioninterface region is defined at the absorber region by the point at whichthe atomic fraction of cadmium exceeds 0.05 in energy dispersive x-rayspectroscopy scans of cross section of the cell and defined at thebuffer region by the point at which the atomic fraction of indium andselenium is less than 0.05 and preferably the atomic fraction of copperis less than 0.10. The atomic fraction is based on total atomic amountsof copper, indium, gallium, selenium, cadmium, sulfur, and oxygen. Thegrain size of the cadmium sulfide grains preferably is less than 50 nm,more preferably less than 30 nm, and most preferably less than 20 nmAccording to this embodiment, the interface region is less than 10 nm,preferably less than 8 nm in thickness.

Atomic fraction can be determined from transmission electron microscope(TEM) line scans using energy dispersive x-ray spectroscopy (EDS).Samples for TEM analysis can be prepared by focused ion beam (FIB)milling using a FEI Strata Dual Beam FIB mill equipped with a Omniprobelift-out tool. TEM analysis can be performed for example on a FEI TecnaiTF-20XT FEGTEM equipped with a Fischione high angle annular dark field(HAADF) scanning TEM (STEM) detector and EDAX EDS detector. Theoperating voltage of the TEM can be level suitable for the equipment forexample voltages of about 200 keV are useful with the equipmentmentioned above.

Spatially-resolved EDS line scans can be acquired in STEM mode fromHAADF images. From the 100 nm long line scan, 50 spectra can be acquiredusing 2 nm spot-to-spot resolution and a STEM probe size of ˜1 nm Thefull scale spectra (0-20 keV) can be converted into elementaldistribution profiles because peak intensity (number of integrated peakcounts after background removal) is directly proportional to theconcentration (in weight percent) of a certain element. Weight percentis then converted to atomic percentage based on mole weight of theelement. Note that as a skilled worker understands peak intensity iscorrected for the detector response and the other sample dependentfactors. These adjustments are typically made based on manufacturer'sparameters for their EDS equipment or other suitable referencestandards. Grain size can be determined by standard analysis of TEMbright and dark field images.

The cadmium sulfide layer may contain a small amount of impurities. Thecadmium sulfide layer preferably consists essentially of cadmium,sulfur, oxygen and copper. Preferably the atomic fractions of cadmiumand sulfur are at least 0.3, the atomic fraction of oxygen is at least0.2 and the atomic fraction of copper is less than 0.15, more preferablyless than 0.10.

The atmosphere for sputtering is preferably an inert gas such as argon,helium or neon. The substrate may be placed at a predetermined distancefrom and orientation relative to the target(s). In some modes ofpractice, this distance can be varied during the course of thedeposition, if desired. Typically, the distance is in the range fromabout 50 millimeters (mm) to about 100 mm. Preferably, the distance isfrom about 60 mm to about 80 mm. Prior to starting deposition, thechamber typically is evacuated to a suitable base pressure. In manyembodiments, the base pressure is in the range from about 1×10⁻⁸ Torr toabout 1×10⁻⁶ Torr.

Conveniently, many modes of practice may be carried out at a temperaturein the range of from about 20° C. to about 30° C. Conveniently, manymodes of practice may be carried out under ambient temperatureconditions. Of course, cooler or warmer temperatures may be used to helpcontrol deposition rate, deposition quality, or the like. The depositionmay be carried out long enough to provide a layer of n-type materialhave a desired thickness, uniformity, and/or the like.

Optional window region 2526 which may be a single layer or formed frommultiple sublayers, can help to protect against shunts. Window region 26also may protect buffer region 28 during subsequent deposition of the TCregion 30. The window region 26 may be formed from a wide range ofmaterials and often is formed from a resistive, transparent oxide suchas an oxide of Zn, In, Cd, Sn, combinations of these and the like. Anexemplary window material is intrinsic ZnO. A typical window region 26may have a thickness in the range from about 1 nm to about 200 nm,preferably about 10 nm to about 150 nm, more preferably about 80 toabout 120 nm.

The TCO region 30, which may be a single layer or formed from multiplesublayers, is electrically coupled to the buffer region 28 to provide atop conductive electrode for article 10. In many suitable embodiments,the TCO region 30 has a thickness in the range from about 10 nm to about1500 nm, preferably about 150 nm to about 200 nm. As shown, the TCOregion 30 is in direct contact with the window region 26, but one ormore intervening layers optionally may be interposed for a variety ofreasons such as to promote adhesion, enhance electrical performance, orthe like.

A wide variety of transparent conducting oxides; very thin conductive,transparent metal films; or combinations of these may be incorporatedused in forming the transparent conductive region 30. Transparentconductive oxides are preferred. Examples of such TCOs includefluorine-doped tin oxide, tin oxide, indium oxide, indium tin oxide(ITO), aluminum doped zinc oxide (AZO), zinc oxide, combinations ofthese, and the like. In one illustrative embodiment, TCO region 30 has adual layer construction in which a first sublayer proximal to the bufferincorporates zinc oxide and a second sublayer incorporates ITO and/orAZO. TCO layers are conveniently formed via sputtering or other suitabledeposition technique.

The optional electrical grid collection structure 40 may be depositedover the TCO region 30 to reduce the sheet resistance of this layer. Thegrid structure 40 preferably incorporates one or more of Ag, Al, Cu, Cr,Ni, Ti, Ta, TiN, TaN, and combinations thereof. Preferably the grid ismade of Ag. An optional film of Ni (not shown) may be used to enhanceadhesion of the grid structure to the TCO region 30. This structure canbe formed in a wide variety of ways, including being made of a wire meshor similar wire structure, being formed by screen-printing, ink-jetprinting, electroplating, photolithography, and metallizing thru asuitable mask using any suitable deposition technique.

A chalcogenide based photovoltaic cell may be rendered less susceptibleto moisture related degradation via direct, low temperature applicationof suitable barrier protection 34 to the top 12 of the photovoltaicarticle 10. The barrier protection may be a single layer or multiplesublayers. As shown the barrier does not cover the electrical gridstructure but a barrier that covered such grid may be used instead or inaddition to the barrier shown.

EXAMPLE Example 1

A photovoltaic cell is prepared as follows. A stainless steel substrateis provided. A niobium and molybdenum backside electrical contact isformed on substrate by sputtering. A copper indium gallium selenideabsorber is formed by a 1-stage evaporation process where copper,indium, gallium and selenium are evaporated simultaneously from effusionsources onto the stainless steel substrate, which is held at ˜550° C.,for ˜80 minutes. This process results in an absorber stoichiometry of˜Cu(In_(0.8)Ga_(0.2))Se₂

A cadmium sulfide layer is radio frequency (re sputtered from a CdStarget (99.9+% purity) at 160 watts in the presence of argon and varyingpressures as shown in Table 1.

The temperature of the substrate is maintained at ≦35° C. and the targetto substrate distance is ˜90 mm The approximate thickness of this layeris as noted in Table 1.

On the cadmium sulfide layer, an i-ZnO and Al-doped ZnO was depositedvia rf-sputtering. A front side electrical collection grid was depositedon Al-doped ZnO.

Cells made substantially according to the above procedure are tested forefficiency by measuring current-voltage characteristics underillumination. As is shown, efficiency peaked at about 0.1 millibar (10Pa).

TABLE 1 CdS Sputtering thickness, Number of Mean pressure, mbar nmsamples efficiency, % 0.002 15 12 6.8 0.002 120 3 6.9 0.01 15 3 6.7 0.0215 3 7.7 0.05 15 3 8.2 0.07 15 3 8.1 0.10 15 32 9.5 0.14 15 4 7.5

Example 2

Cross sections of photovoltaic cells which were prepared substantiallyaccording to the method of Example 1 are prepared by focused ion beam asdescribed in the Detailed Description above. See above

FIGS. 2 and 3 show the atomic fraction from EDS of a cell made on acopper indium gallium selenide based absorber on which cadmium sulfideis sputtered from a 99.9% purity cadmium sulfide target in an argonatmosphere as described above. FIG. 2 shows the atomic fraction of acell made by the present method by sputtering at 0.1 millibar. FIG. 3shows the atomic fraction of a comparative cell made by sputtering at0.002 millibar. This shows that interdiffusion is limited to about a 10nm range in the samples formed at higher pressures.

Example 3

Example 1 is repeated using an absorber made by selenization (usingelemental selenium) of a sputtered layer containing the elements copper,indium and gallium and varying the pressure during CdS sputtering asshown in Table 2. The absorber formation process proceeds as follows:copper, indium and gallium are deposited on a stainless steel substrate,onto which niobium and molybdenum had been previously deposited, viasputtering from either elemental targets or a target made of an alloy ofcopper, indium and gallium. Elemental selenium is evaporated onto thecoated substrates with the substrate temperature maintained at ≦100° C.This coated substrate is then heated to cause selenization of thecopper, indium, gallium precursor.

TABLE 2 CdS Sputtering thickness, Number of Mean pressure, mbar nmsamples efficiency, % 0.09 15 1 5.9 0.10 15 29 6.5 0.12 15 1 4.9

1. A method comprising forming a chalcogenide based absorber layer on asubstrate forming a buffer layer comprising cadmium and sulfur on theabsorber by sputtering at a working pressure of from 0.08 to 0.12 mbar.2. The method of claim 1 wherein the atmosphere is inert.
 3. The methodof claim 1 wherein sputtering is from a target of cadmium and sulfur orcompounds thereof.
 4. The method of claim 1 wherein an interface havinga thickness of less than 10 nm is formed between the absorber layer andthe buffer layer wherein the interface is defined on one side by thepoint at which the atomic fraction of cadmium exceeds 0.05 in energydispersive x-ray spectroscopy scans of cross section of the cell anddefined on a second side by the point at which the atomic fraction ofindium and selenium is less than 0.05 in energy dispersive x-rayspectroscopy scans of cross section of the cell.
 5. The method of claim4 wherein the interface is further defined on the second side by theatomic fraction of copper being less than 0.10.
 6. The method of claim 1wherein the substrate is flexible and bears a backside electricalcontact.
 7. The method of claim 1 further comprising forming atransparent conductive layer over the buffer layer.
 8. The method ofclaim 7 further comprising forming a window layer between thetransparent conductive layer and the buffer layer.
 9. The method ofclaim 7 comprising forming an electrical connection grid on thetransparent conductive layer.
 10. The method of claim 7 comprisingproviding a barrier layer over the transparent conductive layer.
 11. Themethod of claim 9 comprising providing a barrier layer over thetransparent conductive layer and the grid.
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